Plastics have become increasingly popular as replacement materials for glass and metal packaging materials. Advantages of plastic packaging materials over glass packaging materials include lighter weight, decreased breakage and decreased cost. Unlike metal packaging materials, plastic packaging materials can be used to form re-closeable containers. Notwithstanding the above, common plastic packaging materials, for example, polyesters, polyolefins and polycarbonates, tend to be gas permeable and can be problematic if employed to package oxygen-sensitive items, such as foodstuffs, chemicals or pharmaceuticals and/or carbonated beverages.
The extent to which oxygen can permeate a particular plastic packaging material is typically expressed as the oxygen permeability constant. The oxygen permeability constant (herein referred to as "P(O.sub.2)") of such plastic packaging materials, which quantifies the amount of oxygen which can pass through a film or coating under specified conditions, is generally expressed in units of cubic centimeter-mil/100 inches.sup.2 /atmosphere/day. Specifically, this is a standard unit of permeation measured as cubic centimeters of oxygen permeating through a sample of packaging material which is 100 square inches (645 square centimeters) in area and 1 mil (25.4 microns) thick, over a 24 hour period, under a partial pressure differential of one atmosphere, at a specified temperature and relative humidity ("R.H."). As used herein, unless otherwise stated, P(O).sub.2 values are reported at 25.degree. C. at 50-55 percent R.H.
Many foodstuffs, beverages, chemicals and pharmaceuticals are susceptible to oxidation which can cause discoloration and/or spoilage. Hence, such items must be protectively packaged to prevent exposure to oxygen. Moreover, carbonated beverages must be stored in sealed containers to prevent escape of gaseous carbon dioxide which could render the beverage unacceptably "flat". Since oxygen and carbon dioxide can readily permeate through many of the plastic packaging materials commonly used in the packaging industry, items stored in conventional plastic containers have a significantly reduced shelf life as compared to the shelf life of those items when packaged in glass or metal containers.
Some specific examples of particularly oxygen-sensitive items include perishable foodstuffs and beverages, such as tomato-based products, for example, catsup, tomato sauces and tomato pastes, fruit and vegetable juices, and malt beverages, for example, beer, ale and malt liquor. Exposure to even minute amounts of oxygen over a relatively short period of time can adversely affect the color and taste of such products. Some specific examples of carbonated beverages, the shelf life of which may be seriously reduced if packaged in conventional plastic containers, include malt beverages, soft drinks, sparkling water, sparkling wine and the like.
One of the most common plastic packaging materials used in the food and beverage industry is poly(ethylene terephthalate) (hereinafter referred to as "PET"). Notwithstanding widespread use in the industry, PET has a relatively high P(O.sub.2) value (i.e., approximately 6.0). For this reason, the food and beverage industry has sought to improve the P(O.sub.2) value of PET. It should be understood that, although P(O.sub.2) values refer to the permeability of oxygen through a film or coating, lowering the P(O.sub.2) value not only improves oxygen barrier properties, but can improve carbon dioxide barrier properties as well.
Generally, there are two methods known in the art for improving the P(O.sub.2) of a plastic packaging material. The plastic itself can be chemically and/or physically modified. This method is typically expensive and can create problems during recycling. Alternatively, the plastic packaging material can be coated with a gas barrier material, as by applying thereto a gas barrier coating composition or a gas barrier film. The latter method is commercially more attractive than the former because it is typically more cost effective and creates few, if any, recycling problems.
Numerous gas barrier coating compositions have been disclosed in the prior art. For example, polyepoxide-polyamine based gas barrier coating compositions having low P(O.sub.2) values are disclosed in commonly-owned U.S. Pat. Nos. 5,006,361; 5,008,137 5,300,541; 5,006,381; and WO 95126997. Also known in the art are polyepoxide-polyamine based gas barrier coatings having very low P(O.sub.2) values which further comprise platelet-type fillers, such as silica and mica, having a specified particle size distribution. The presence of the platelet-type fillers in the gas barrier coating compositions provides a plastic packaging material having improved barrier properties while maintaining high gloss appearance properties. The above-mentioned coating compositions generally have found commercial acceptance as gas barrier coatings for polymeric containers.
For certain applications, the gas barrier packaging material must meet stringent chemical resistance requirements. For example, fruit juices typically are pasteurized at a temperature of 180.degree. F. to 190.degree. F. (82.degree. C. to 87.degree. C.) prior to filling. The plastic containers formed from gas barrier packaging material are filled with the hot product. This process is commonly referred to as a "hot-fill" process. During the hot-fill process, the gas barrier coating (which had been applied to the plastic container to form a gas barrier packaging material) can be contacted with hot fruit juices which often are highly acidic. For these hot-fill applications, the gas barrier packaging material must not only provide gas barrier properties, but must be chemically resistant as well.
Hydroxy-substituted aromatic compounds are well-known in the art as catalysts for the curing reaction between polyamines and polyepoxides. See Accelerated Amine Curing of Epoxy Resins, L. H. Gough et al., Research Department, Cray Valley Products, Ltd., reprinted in 43 J.O.C.C.A. 409-18, June 1960 and references cited above. It is not known, however, to employ such compounds in gas barrier coating compositions for the enhancement of gas barrier properties. Moreover, these hydroxy-substituted aromatic compounds are not known for use in thermoplastic polyamine-polyepoxide based gas barrier coating compositions.
The chemical resistance of the aforementioned polyamine-polyepoxide based gas barrier coatings can be improved by reducing the amine:epoxy ratio in the composition. However, a reduction in the amount of polyamine in the composition, which can result in improved chemical resistance, can also result in a packaging material having reduced gas barrier properties. In view of the foregoing, clearly a need exists in the food and beverage packaging industry for a chemically resistant packaging material having improved gas barrier properties.